Polyacetal resin composition

ABSTRACT

A polyacetal resin composition having high rigidity and also excellent in dimensional stability and creep characteristics is provided. A polyacetal resin composition prepared by blending (A) 99.9 to 90 parts by weight of a linear polyacetal resin having a melt index of 1 to 50 g/min obtained by copolymerizing (a) 99.5 to 97.5% by weight of trioxane and (b) 0.5 to 2.5% by weight of a compound selected from a mono-functional cyclic ether compound and a mono-functional cyclic formal compound, with (B) from 0.1 to 10 parts by weight of a branched or crosslinked polyacetal resin having a melt index of 0.1 to 10 g/min obtained by copolymerizing (a) 99.49 to 95.0% by weight of trioxane, (b) 0.5 to 4.0% by weight of a compound selected from a mono-functional cyclic ether compound and mono-functional cyclic formal compound and (c) 0.01 to 1.0% by weight of a poly-functional glycidyl ether compound with the number of functional groups of 3 to 4, in which (A) and (B) are selected so that the ratio between the melt index of (A) and the melt index of (B) can satisfy the relation of the following formula: 
 
0.02≦ MI   B   /MI   A ≦1.5   (1) 
 
(where MI A  is a melt index of (A) and MI B  is a melt index of (B))

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a polyacetal resin composition havinghigh rigidity and being excellent in dimensional stability and creepcharacteristics.

PRIOR ARTS

A Polyacetal resin has excellent properties in mechanical property,thermal property, electric property, slidability, moldability etc. andhas been widely used as mainly structural materials and/or functionalparts, etc. in electric instruments, car parts, precision machine parts,etc. However, with the increasing application area of polyacetal resin,the resin is required to satisfy more and more sophisticated,complicated and specialized requirements. By way of example, there is ademand for a material having much more enhanced rigidity, surfacehardness, slidability, etc. while maintaining various properties ofpolyacetar resin itself such as excellent moldability and appearance.For meeting such a demand, the present inventors have proposed apolyacetal resin composition by blending a polyacetal resin with apolyacetal copolymer having branched and/or bridged structures inJP2002-3694A. However, according to their subsequent detailinvestigation, the polyacetal resin composition has a enhanced rigidity,surface hardness, and slidability, but their dimensional stability andcreep characteristics are unsatisfactory.

Wherein, dimensional stability is an important property for mechanicalparts such as gears. With poor dimensional stability, elevatedtemperature within machine results in post-shrinkage of the presentcomposition, and thereby fails to engage a gear. It follows that torquetransmission fail. To solve these problems, one of general method isthat the composition anneals for a long time below the melting point ofthe resin after molding, and stabilizes the crystal condition ofpolyacetal resin, and thereby increases the accuracy of the size. Themethod needs high production costs and causes durability loss of themolded articles because of the defects resulted inside the moldedarticles due to the rapid crystal shrinkage just after molding.

Also, in mechanical parts, etc., as there is a need for reducing thedeformation under specified loading, and expanding a long life for usein many cases, creep characteristics is also one of importantproperties. Therefore, there is a further demand for improvement ofcreep characteristics as well as dimensional stability.

The polyacetal resin composition by blending two or more polyacetalresins having a different properties and structures other than abovecomposition is disclosed in several specifications, for example,JP2001-2886A, JP2001-2885A, JP9-241476A, JP5-279551A, JP4-108848A,JP3-263454A, JP3-756A, JP1-20258A, JP59-129247A, JP50-30949A,JP49-58145A, JP48-97955A, JP48-30749A, JP47-14249A, etc. are known.

However, any polyacetal resin material having high rigidity andexcellent in dimensional stability and creep characteristics has notbeen disclosed in these specifications.

DISCLOSURE OF THE INVENTION

A purpose of the present invention is to solve the above problems and toprovide a polyacetal resin composition having high rigidity and beingexcellent in dimensional stability and creep characteristics.

For achieving the above object, the present inventors have carried out adetail investigation in order to attain the above-described purpose. Asa result, they have found that a blend of two polyacetal resins havingspecified structures and properties allows to provide materialssatisfying all of high rigidity, dimensional stability, and creepcharacteristics, whereupon the present invention has been achieved.

That is, the present invention relates to a polyacetal resin compositionprepared by blending

-   -   (A) from 99.9 to 90 parts by weight of a linear polyacetal resin        having a melt index of 1 to 50 g/min obtained by        copolymerizing (a) 99.5 to 97.5% by weight of trioxane and (b)        0.5 to 2.5% by weight of a compound selected from a        mono-functional cyclic ether compound and a mono-functional        cyclic formal compound, with    -   (B) from 0.1 to 10 parts by weight of a branched or crosslinked        polyacetal resin having a melt index of 0.1 to 10 g/min obtained        by copolymerizing (a) 99.49 to 95.0% by weight of trioxane, (b)        0.5 to 4.0% by weight of a compound selected from a        mono-functional cyclic ether compound and a mono-functional        cyclic formal compound, and (c) 0.01 to 1.0% by weight of a        poly-functional glycidyl ether compound with the number of        functional groups of 3 to 4, in which    -   the linear polyacetal resin (A) and the branched or crosslinked        polyacetal resin (B) are selected so that the ratio between the        melt index of the linear polyacetal resin (A) and the melt index        of the branched or crosslinked polyacetal resin (B) can satisfy        the relation of the following formula (1):        0.02≦MI _(B) /MI _(A)≦1.5   (1)        (wherein MI_(A) is a melt index of the linear polyacetal        resin (A) and MI_(B) is a melt index of the branched or        crosslinked polyacetal resin (B)).

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be explained in detail. First, the linearpolyacetal resin (A) used in the present invention is obtained bycopolymerizing (a) 99.5 to 97.5% by weight of trioxane and (b) 0.5 to2.5% by weight of a compound selected from a mono-functional cyclicether compound and a mono-functional cyclic formal compound, and thelinear polyacetal resin has a melt index of 1 to 50 g/min. Wherein, themelt index as defined herein is measured according to ASTM D-1238 at atemperature of 190 degrees C. under a loading of 2160 g.

The trioxane (a) as the base material for producing the linearpolyacetal resin (A) is a cyclic trimer of formaldehyde, which isgenerally obtained by a reacting of an aqueous solution of formaldehydein the presence of an acid catalyst, and is used after purifying bydistillation etc. It is preferred that the trioxane (a) used for thepolymerization contains as little as possible of impurities such aswater, methanol and formic acid.

The compound (b) selected from mono-functional cyclic ether compoundsand mono-functional cyclic formal compounds using for production of thelinear polyacetal resin (A) by copolymerization with the trioxane (a) isa compound having one cyclic ether unit or cyclic formal unit in onemolecule. The compound (b) includes ethylene oxide, propylene oxide,butylene oxide, epichlorohydrin, epibromohydrin, styrene oxide, oxetane,3,3-bis(chloromethyl)oxetane, tetrahydrofuran, trioxepane, 1,3-dioxolan,ethylene glycol formal, propylene glycol formal, diethyleneglycolformal, triethyleneglycol formal, 1,4-butanediol formal, 1,5-pentanediolformal, 1,6-hexanediol formal etc. Among them, at least one compoundselected from the group consisting of ethyleneoxide, 1,3-dioxolan,1,4-butanediol formal and diethylene glycol formal is preferably used.

In the linear polyacetal resin (A) used in the present invention,copolymerization rate of the compound (b) selected from thesemono-functional cyclic ether compounds and mono-functional cyclic formalcompounds is from 0.5 to 2.5% by weight to (a) 99.5 to 97.5% by weightof trioxane. If the copolymerization rate of the compound (b) is lowerthan the range, the polyacetal resin composition having demandedexcellent dimensional stability cannot be easily obtained. On the otherhand, if the copolymerization rate of the compound (b) is higher thanthe range, the polyacetal resin composition having demanded highrigidity, dimensional stability and creep characteristics cannot beeasily obtained. Both of the cases are undesirable. Especiallypreferable copolymerization rate of the compound (b) is from 0.7 to 2.0%by weight.

The linear polyacetal resin (A) used in the present invention isgenerally obtained, for example, by a method of bulk polymerizationusing a cationic polymerization catalyst, where an appropriate amount ofa molecular-weight regulator is added thereto. Examples of the molecularweight regulator include low molecular acetal compounds having alkoxygroups such as methylal, methoxy methylal, dimethoxymethylal, trimethoxymethylal and oxymethylene di-n-butyl ether, alcohols such as methanol,ethanol and butanol, ester compounds, acid compounds, and water. Amongthese compounds, the low molecular acetal compounds having alkoxy groupsare particularly preferable.

Also, examples of the cationic polymerization catalyst include leadtetrachloride, tin tetrachloride, titanium tetrachloride, aluminumtrichloride, zinc chloride, vanadium trichloride, antimony trichloride,phosphorus pentafluoride, antimony pentafluoride, boron trifluoride,boron trifluoride coordination compounds such as borontrifluoride-diethyl ethelate, boron trifluoride-dibutyl ethelate, borontrifluoride-dioxanate, boron trifluoride-acetic anhydrate and borontrifluoride-triethylamine complex compounds, inorganic and organic acidssuch as perchloric acid, acetyl perchlorate, t-butyl perchlorate,hydroxyacetic acid, trichloroacetic acid, trifluoroacetic acid andp-toluene sulfonic acid, complex salt compounds such as triethyl oxoniumtetrafluoroborate, triphenyl methyl hexafluoroantimonate, allyldiazoniumhexafluorophosphate and allyldiazonium tetrafluoroborate, alkyl metalsalts such as diethyl zinc, triethyl aluminum and diethyl aluminumchloride, heteropoly acid and isopoly acid, etc. Among these compounds,boron trifluoride and boron trifluoride coordination compounds such asboron trifluoride-diethyl ethelate, boron trifluoride-dibutyl ethelate,boron trifluoride-dioxanate, boron trifluoride-acetic anhydrate andboron trifluoride-triethylamine complex compounds are especiallypreferable. Such a catalyst can be used after it may be diluted with anorganic solvent, etc. and then used.

There is no particular limitation for the polymerizer in the productionof the linear polyacetal resin (A) used in the present invention. Knownapparatuses may be used and in particular, a continuous polymerizerhaving two axles with paddles etc. is suitably used. It is preferred tokeep the polymerization temperature at 65 to 135 degrees C.Deactiviation of the catalyst after polymerization is carried out byadding a basic compound or an aqueous solution thereof to a reactionproduct discharged from the polymerizer after the polymerizationreaction or to a reaction product in the polymerizer.

Examples of the basic compound for neutralizing and deactivating thepolymerization catalyst include ammonia, or amines such astriethylamine, tributylamine, triethanolamine and tributanolamine, orhydroxide salts of alkali metal or alkaline earth metal, and other knowndeactivators of the catalyst. It is preferred that, after thepolymerization, an aqueous solution thereof is added to the productquickly to deactivate. After such a polymerization and a deactivation,if necessary, washing, separation/recovery of unreacted monomers,drying, etc. may be carried out by conventional methods.

The linear polyacetal resin (A) used in the present invention isobtained by the above method, the melt index thereof is adjusted from 1to 50 g/min. If the melt index is lower than the range, the resincomposition having high rigidity and high dimensional stability cannotbe easily obtained by blending the linear polyacetal resin (A) withbranched or bridged polyacetal resin (B) as described later. On theother hand, if the melt index is higher than the range, the resincomposition having high rigidity and being excellent in dimensionalstability and creep characteristic cannot be easily obtained. Both ofthe cases are undesirable.

Next, the branched or crosslinked polyacetal resin (B) used in thepresent invention is obtained by copolymerizing (a) 99.49 to 95.0% byweight of trioxane, (b) 0.5 to4.0% by weight of a compound selected froma mono-functional cyclic ether compound and a mono-functional cyclicformal compound, and (c) 0.01 to 1.0% by weight of a poly-functionalglycidyl ether compound with the number of functional groups of 3 to 4,in which

-   -   the branched or crosslinked polyacetal resin (B) has a melt        index of from 0.1 to 10 g/min.

Wherein trioxane (a) and compound (b) selected from mono-functionalcyclic ether compounds and mono-functional cyclic formal compounds,which are used in the production of the branched or crosslinkedpolyacetal resin (B), are compounds as described in detail in theillustration of the linear polyacetal resin (A). The compound (b) usedin the production of the branched or crosslinked polyacetal resin (B) issame or different compared to the compound (b) used in the production ofthe linear polyacetal resin (A).

Also, the poly-functional glycidyl ether compound (c) with the number offunctional groups of 3 to 4, which are used in the production of thebranched or crosslinked polyacetal resin (B), refers to the compoundhaving 3 to 4 glycidyl ether units in one molecule. The poly-functionalglycidyl ether compound (c) is any compound selected from abovecompounds without limiting. For example, at least one compound selectedfrom the group consisting of trimethylolpropane triglycidyl ether,glycerol triglycidyl ether and pentaerythritol tetraglycidyl ether ispreferably used.

The branched or bridged polyacetal resin (B) used in the presentinvention is obtained by copolymerizing (a) 99.49 to 95.0% by weight ofabove trioxane, (b) 0.5 to 4.0% by weight of above compound, and (c)0.01 to 1.0% by weight of above poly-functional glycidyl ether compoundwith the number of functional groups of 3 to 4. If the copolymerizationratio of the compound (b) and the poly-functional glycidyl ethercompound (c) is lower or higher than the range, the polyacetal resincomposition having high rigidity with dimensional stability and creepcharacteristics cannot be easily obtained by blending the branched orcrosslinked polyacetal resin (B) with the linear polyacetal resin (A).For the branched or crosslinked polyacetal resin (B), copolymerizationratio of the compound (b) is especially preferably from 0.7 to 3.0% byweight, copolymerization ratio of the poly-functional glycidyl ethercompound (c) is especially preferably from 0.02 to 0.5% by weight.

The branched or bridged polyacetal resin (B) used in the presentinvention is generally obtained, similar to the linear polyacetal resin(A), for example, by a method of cationic polymerization using acationic polymerization catalyst, where an appropriate amount of amolecular-weight regulator is added thereto. Also, polymerizer,condition of the polymerization, deactivation of catalyst afterpolymerization and subsequent post-treatment, etc. can be conductedaccording to the method for producing the linear polyacetal resin (A).

The branched or bridged polyacetal resin (B) obtained by the abovemethod and used in the present invention is adjusted to a melt index offrom 0.1 to 10 g/min. If the melt index is lower than the range, theresin composition having demanded dimensional stability and creepcharacteristics cannot be easily obtained. On the other hand, if themelt index is higher than the range, the resin composition having highrigidity and excellent in dimensional stability and creepcharacteristics cannot be easily obtained.

The polyacetal resin composition of the present invention ischaracterized by blending

-   -   (A) 99.9 to 90 parts by weight of a linear polyacetal resin and    -   (B) 0.1 to 10 parts by weight of a branched or crosslinked        polyacetal resin,    -   wherein the linear polyacetal resin (A) and the branched or        crosslinked polyacetal resin (B) are selected so that the ratio        between the melt index of the linear polyacetal resin (A) and        the melt index of the branched or crosslinked polyacetal        resin (B) can satisfy the relation of the following formula (1):        0.02≦MI _(B) /MI _(A)≦1.5   (1)        (wherein MI_(A) is a melt index of the linear polyacetal        resin (A) and MI_(B) is a melt index of the branched or        crosslinked polyacetal resin (B)).

If the amount of the branched or crosslinked polyacetal resin (B) to beblended is lower than the range, the resin composition having highrigidity and excellent in dimensional stability and creepcharacteristics cannot be easily obtained. On the other hand, if theamount is higher than the range, the resin composition having demandeddimensional stability and creep characteristics cannot be easilyobtained.

Also, if the ratio of the melt index MI_(B) of the branched orcrosslinked polyacetal resin (B) to the melt index MI_(A) of the linearpolyacetal resin (A), MI_(B)/MI_(A), is less than 0.02, the resincomposition having demanded high rigidity as well as dimensionalstability and creep characteristics cannot be easily obtained. On theother hand, the ratio of the melt index MI_(B)/MI_(A) is over 1.5, theresin composition having high rigidity and excellent in dimensionalstability cannot be easily obtained.

Also, in the invention, the melt index of the linear polyacetal resin(A), the melt index of the branched or crosslinked polyacetal resin (B)and the blending ratio of them are especially preferably controlled sothat the melt index of a polyacetal resin composition in which thebranched or crosslinked polyacetal resin (B) is blended with the linearpolyacetal resin (A) can satisfy the relation of the following formula(2) relative to the melt index of the linear polyacetal resin (A):0.7≦MI _(A) /MI _(AB)≦1.4   (2)(wherein MI_(A) is a melt index of the linear polyacetal resin (A) andMI_(AB) is a melt index of the polyacetal resin composition).

If the ratio MI_(A)/MI_(AB), which is the ratio of the melt index(MI_(A)) of the linear polyacetal resin (A) to the melt index (MI_(AB))of the polyacetal resin composition is lower or higher than the range,the polyacetal resin composition having high rigidity with dimensionalstability and characteristics cannot be easily obtained.

The polyacetal resin composition of the present invention is basicallyprepared by melt mixing the linear polyacetal resin (A) and the branchedor crosslinked polyacetal resin (B). The process condition of the meltmixing is preferably at a temperature of from 180 to 270 degrees C. andat least 30 seconds. An illustrative embodiment of the preparationmethod is not limiting, the method may be applied known equipments andmethods, for example, mixing the required components using one-axle ortwo-axle extruders or other melt-mixer, and producing pellets formolding, etc.

The polyacetal resin composition of the present invention may preferablybe blended with various stabilizers selected as necessary. Examples ofthe stabilizers include at least one compound selected from hinderedphenol type compounds, nitrogen-including compounds, hydroxides ofalkaline or alkaline earth metals, inorganic salts and carboxylates.Further, one or more common additives for thermoplastic resin, such ascoloring agents e.g. dye, pigment etc., lubricants, releasing agents,antistatic agents, surfactants, or organic polymer materials, andinorganic or organic fillers in a form of fiber, powder and plate may beadded as necessary as far as the object and effect of the presentinvention are not hindered.

THE EFFECT OF THE INVENTION

The polyacetal resin composition of the present invention has highrigidity, dimensional stability, and creep characteristics, and is alsoexcellent in surface hardness, and slidability. The polyacetal resincomposition can suitably be used as structural materials and/orfunctional parts, etc. in electric instruments, car parts, precisionmachine parts, etc.

EXAMPLES

Now, the present invention will be described in detail by reference tothe Examples, which are not intended to limit the present invention.Various assessments were conducted according to the following methods.

[Melt Index]

Melt index (Ml) was measured according to ASTM D-1238 at 190 degrees C.,under a loading of 2160 g.

[Formulation of Copolymer]

The formulation of copolymer was identified using ¹H-NMR mesurement withhexafluoroisopropanol d₂ as a solvent.

[Tensile Strength]

Test pieces for ISO were molded and the tensile strength was measuredaccording to ISO method.

[Dimensional Change]

Tensile test pieces for ISO were molded, and test pieces were stoodwithin the conditioned room at a temperature 23 degrees C., and humidityof 50% for 24 hour, and then sizes of test pieces were measured. Afterthe measurement, test pieces were treated at 70 degrees C. for 5 hours.Again, after the test pieces were stood within the conditioned room for24 hours, sizes of test pieces were measured, the difference initialsizes and sizes after treatment was the dimensional change.

[Fracture Life]

Test pieces for ISO was molded, and then fracture life was measuredusing a tensile creep tester with lever. According to the level offracture life, test pieces were assessed as excellent (E) good (G) andno good (NG).

Production Examples 1 to 9 and Comparative Production Examples 1 to 9

A continuous mixing reactor constituted from a barrel having a jacketfor passing a heating (or cooling) medium at outside and having a shapeof the cross section where two circles are partially overlapped, androtating shafts equipped with a paddle was used, and trioxane (a),compound (b) selected from mono-functional cyclic ether compounds andmono-functional cyclic formal compounds, and poly-functional glycidylether compound (c) were added thereinto in a ratio shown in Tables 1 and2 while each of two rotating shafts having a paddle was rotated at 150rpm. Then, methylal was continuously fed as the molecular-weightregulator, and as the catalyst, boron trifluoride was added in an amountof 0.005% by weight to the trioxane, and the uniform mixture wasbulk-polymerized. The reaction product discharged from the polymerizerwas immediately passed through a grinder and added to an aqueoussolution containing 0.05% by weight of triethylamine at 60 degrees C. todeactivate the catalyst. After separation, washing and drying, a crudepolyacetal copolymer (a linear polyacetal resin and a branched orcrosslinked polyacetal resin) was obtained.

Then, to 100 parts by weight of the crude polyacetal copolymer wereadded 3% by weight of a 5% by weight aqueous solution of triethylamineand 0.3% by weight of pentaerythrityl-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate), followed bysubjecting to melting and kneading at 210 degrees C. in a twin extruderto remove unstable parts. Polyacetal resins in forms of pellet (linearpolyacetal resins A1-A3, a1-a4, and branched or bridged polyacetalresins B1-B6, b1-b5) were obtained, and then the polyacetal resins wereused in the preparation of the polyacetal resin compositions.

The formulation and melt index of these polyacetal resins are shown inTables 1 and 2.

Examples 1-9

The linear polyacetal resin and the branched or crosslinked polyacetalresin within the scope of the present invention, which are obtainedaccording to the method of the above production examples, are blended inthe rate of both resins shown in Table 3, and in the rate of the meltindex within the present invention, followed by subjecting to meltingand kneading at 210 degrees C. in a twin extruder to get pellets of thepolyacetal resin composition. Their properties were estimated accordingto the above method. The results are shown in Table 3.

The linear polyacetal resin and the branched or crosslinked polyacetalresin, which are composed of the polyacetal resin composition, the ratioof the blend, and the ratio of the melt index are all within the presentinvention. Therefore, the resulting polyacetal resin composition hashigh rigidity, dimensional stability, and creep characteristics. Also,in any compositions, the property of high rigidity is satisfactory.

Comparative Examples 1-15

As shown in Table 3, at least one of a linear polyacetal resin, abranched or crosslinked polyacetal resin, a rate of a blend, and a rateof a melt index were changed other than the condition of the presentinvention to blend the linear polyacetal resin and the branched orcrosslinked polyacetal resin, followed by subjecting to melting andkneading at 210 degrees C. in a twin extruder to get pellets of thepolyacetal resin composition. Their properties were estimated accordingto the above method. The results are shown in Table 3.

Also, comparative examples 14 and 15 show the linear polyacetal resinwithout blending the branched or crosslinked polyacetal resin. If any ofthe linear polyacetal resin and the branched or crosslinked polyacetalresin, which are composed of the polyacetal resin composition, the ratioof the blend, and the ratio of the melt index are other than the regionof the present invention, the polyacetal resin composition having highrigidity with dimensional stability and creep characteristics cannot beeasily obtained.

Then, abbreviations in Tables are as follows.

-   Component (b)-   DO: 1,3-dioxolan-   BF: 1,4-butanediol formal-   Component (c)-   TMPTGE:trimethylol propane triglycidyl ether

PETGE: pentaerythritol tetraglycidyl ether TABLE 1 Polyacetal Trioxane(a) Compound (b) Melt index MI_(A) resin No. (wt %) Kind (wt %) (g/10min) Production Ex. 1 A1 98.3 DO 1.7 2.5 Production Ex. 2 A2 99.0 DO 1.02.5 Production Ex. 3 A3 98.3 BF 1.7 2.4 Comparative Production Ex. 1 a196.6 DO 3.4 2.5 Comparative Production Ex. 2 a2 99.8 DO 0.2 2.5Comparative Production Ex. 3 a3 98.3 DO 1.7 0.5 Comparative ProductionEx. 4 a4 98.3 DO 1.7 95

TABLE 2 Melt Polyacetal Trioxane (a) Compound (b) Compound (c) indexMI_(B) resin No. (wt %) Kind (wt %) Kind (wt %) (g/10 min) ProductionEx. 4 B1 98.2 DO 1.7 TMPTGE 0.1 1.5 Production Ex. 5 B2 98.2 DO 1.7TMPTGE 0.1 0.9 Production Ex. 6 B3 98.2 DO 1.7 TMPTGE 0.1 5.0 ProductionEx. 7 B4 98.9 DO 1.3 TMPTGE 0.1 1.5 Production Ex. 8 B5 98.0 DO 1.7TMPTGE 0.3 0.9 Production Ex. 9 B6 98.2 DO 1.7 PETGE 0.1 1.5 Comparativeb1 98.2 DO 1.7 TMPTGE 0.1 20 Production Ex. 5 Comparative b2 96.8* DO1.7* TMPTGE 1.5* 0 Production Ex. 6 Comparative b3 98.3 DO 1.7 TMPTGE0.005 1.5 Production Ex. 7 Comparative b4 99.7 DO 0.2 TMPTGE 0.1 1.5Production Ex. 8 Comparative b5 96.5 DO 5.0 TMPTGE 0.1 1.5 ProductionEx. 9*Charged amount is shown because the polyacetal resin is insoluble tohexafluoroisopropanol d₂.

TABLE 3 Linear polyacetal resin Branched or closslinked Polyacetal resin(A) polyacetal resin (B) composition Melt Melt Melt index Blending indexBlending index Polyacetal MI_(A) amount Polyacetal MI_(B) amount MI_(AB)Tensile Dimensional resin (g/ (wt. resin (g/ (wt. MI_(B)/ (g/10 MI_(A)/strength change Fracture No. 10 min Pts) No. 10 min) pts) MI_(A) min)MI_(AB) (MPa) (mm) life Examples 1 A1 2.5 97 B1 1.5 3 0.6 2.4 1.0 69.5−0.03 E 2 A1 2.5 95 B1 1.5 5 0.6 2.3 1.1 70.1 −0.02 E 3 A1 2.5 92 B1 1.58 0.6 2.3 1.1 70.5 −0.04 E 4 A2 2.5 95 B1 1.5 5 0.6 2.3 1.1 71.0 −0.03 E5 A3 2.4 95 B1 1.5 5 0.63 2.2 1.1 69.8 −0.02 E 6 A1 2.5 95 B2 0.9 5 0.362.2 1.1 70.9 −0.03 E 7 A1 2.5 95 B4 1.5 5 0.6 2.3 1.1 70.3 −0.03 E 8 A12.5 95 B5 0.9 5 0.36 2.1 1.2 70.4 −0.04 E 9 A1 2.5 95 B6 1.5 5 0.6 2.11.2 70.2 −0.02 E Comparative Examples 1 a1 2.5 95 B1 1.5 5 0.6 2.4 1.064.8 −0.09 NG 2 a2 2.5 95 B1 1.5 5 0.6 2.2 1.1 71.5 −0.10 E 3 a3 0.5 95B1 1.5 5 3.0 0.8 0.6 62.3 −0.13 E 4 a4 95 95 B1 1.5 5 0.016 78 1.2 63.2−0.07 NG 5 A1 2.5 95 b1 20 5 8.0 4.8 0.5 65.4 −0.08 NG 6 A1 2.5 95 b2 05 0 1.7 1.5 68.3 −0.09 NG 7 A1 2.5 95 b3 1.5 5 0.6 2.4 1.0 62.9 −0.09 NG8 A1 2.5 95 b4 1.5 5 0.6 2.4 1.0 69.1 −0.08 G 9 A1 2.5 95 b5 1.5 5 0.62.3 1.1 65.9 −0.10 G 10 A1 2.5 99.95 B1 1.5 0.05 0.6 2.5 1.0 64.3 −0.09NG 11 A1 2.5 80 B1 1.5 20 0.6 1.7 1.5 70.2 −0.11 NG 12 A1 2.5 95 B3 5.05 2.0 2.7 0.9 64.9 −0.07 G 13 A3 2.4 96 B3 5.0 4 2.1 2.6 0.9 64.2 −0.08G 14 A1 2.5 100 — — — — — — 62.7 −0.09 NG 15 a1 2.5 100 — — — — — — 60.0−0.12 NG

1. A polyacetal resin composition prepared by blending (A) from 99.9 to90 parts by weight of a linear polyacetal resin having a melt index of 1to 50 g/min obtained by copolymerizing (a) 99.5 to 97.5% by weight oftrioxane and (b) 0.5 to 2.5% by weight of a compound selected from thegroup consisting of a mono-functional cyclic ether compound and amono-functional cyclic formal compound, with (B) from 0.1 to 10 parts byweight of a branched or crosslinked polyacetal resin having a melt indexof 0.1 to 10 g/min obtained by copolymerizing (a) 99.49 to 95.0% byweight of trioxane, (b) 0.5 to 4.0% by weight of a compound selectedfrom the group consisting of a mono-functional cyclic ether compound andmono-functional cyclic formal compound and (c) 0.01 to 1.0% by weight ofa poly-functional glycidyl ether compound with the number of functionalgroups of 3 to 4, in which the linear polyacetal resin (A) and thebranched or crosslinked polyacetal resin (B) are selected so that theratio between the melt index of the linear polyacetal resin (A) and themelt index of the branched or crosslinked polyacetal resin (B) cansatisfy the relation of the following formula (1):0.02≦MI _(B) /MI _(A)≦1.5   (1) (where MI_(A) is a melt index of thelinear polyacetal resin (A) and MI_(B) is a melt index of the branchedor crosslinked polyacetal resin (B)).
 2. The polyacetal resincomposition as defined in claim 1, wherein the melt index of the linearpolyacetal resin (A), the melt index of the branched or crosslinkedpolyacetal resin (B) and the blending ratio of them are controlled sothat the melt index of a polyacetal resin composition in which thebranched or crosslinked polyacetal resin (B) is blended with the linearpolyacetal resin (A) can satisfy the relation of the following formula(2) relative to the melt index of the linear polyacetal resin (A):0.7≦MI _(A) /MI _(AB)≦1.4   (2) (where MI_(A) is a melt index of thelinear polyacetal resin (A) and MI_(AB) is a melt index of thepolyacetal resin composition).
 3. The polyacetal resin composition asdefined in claim 1, wherein the poly-functional glycidyl ether compound(c) is at least one selected from the group consisting of trimethylolpropane triglycidyl ether, glycerol triglycidyl ether andpentaerythritol tetraglycidyl ether.
 4. The polyacetal resin compositionas defined in claim 1, wherein the compound (b) is at least materialselected from the group consisting of ethylene oxide, 1,3-dioxolane,1,4-butanediol formal and diethylene glycol formal.